As both of inkjet ink and media have been developed, a high quality image comparative to photographic image quality is achieved by inkjet recording. Particularly, as disclosed in Patent Literatures 1 and 2, an inkjet recording method in which an aqueous ink and porous medium are used becomes popular in terms of its convenience and safety, so that the conventional silver halide method is being replaced with the inkjet recording method in the field of photography.
On the other hand, an aqueous inkjet recording method has become used for inkjet coated paper as well as for plain paper or inkjet matte paper in a commercial printing field. However, in a commercial printing field where a printing cost, speed, and image quality are severely demanded, the aqueous inkjet recording method is limited to use in certain fields such as proof or page wide printing. A method of providing image quality comparable to commercial printing on coated paper (commercial coated paper), which is used for general offset printing or gravure printing, at adequately high speed by the aqueous inkjet recording, has not been achieved yet. When the inkjet printing is performed on common coated paper for commercial printing, ink absorption and wettability are poor, causing many problems, for example, image bleeding and dry failure. Therefore, currently the aqueous inkjet recording method cannot be practically used in the commercial printing field.
Conventionally, to obtain a print having texture of commercial printing paper, such as A2 grade glossy coated paper, A2 grade matte coated paper and art paper by the aqueous inkjet recording, each of these paper sheets is used as a base and a transparent inkjet receiving layer as a surface layer is formed over the base. This technique is practically used in applications such as proof as mentioned above. However, the production cost of the media is extremely high than that of the common printing paper, and eventually, the printing cost thereof is also extremely high. Therefore, it is not at all practical to apply the technique to leaflets distribution and the like.
Moreover, there is another method in which a material having relatively large oil absorption, such as colloidal silica, alumina hydrate or the like, is applied on a base paper so as to have sufficient ink absorption, and calcium carbonate or other pigments are used together, thereby forming a coat layer having texture similar to that of printing paper. In this method, because raw materials such as colloidal silica, alumina hydrate and the like are also expensive, the cost of the media tends to be high at any price. Therefore, the application of this method is currently limited only to proof or the like. It is difficult to apply the method to commercial printing such as leaflets, catalogs and the like, in which priority is given to printing cost.
Particularly, by using silica, alumina hydrate or the like, a medium which has a coat layer having texture similar to that of printing paper easily causes peculiar problems. Namely, in the case of this type of media, in order to obtain high quality image by inkjet printing, a large amount of ink needs to be absorbed in the media similar to standard inkjet paper. Thus, it is necessary to ensure ink absorption by adding a large amount of pigment having relatively large oil absorption in the coat layer. When the large amount of pigment having large oil absorption is contained in the coat layer, ink absorption is improved, but the coat layer is easily embrittled (powder fall). The embrittlement is prevented by increasing the blending ratio of binder, but this inhibits ink absorption, and the object cannot be achieved. One technical point of improving this type of media is how these problems are resolved, as disclosed in Patent Literature 3. However, the media tend to be inferior to the commercial printing paper. Moreover, when this kind of media are applied to the commercial printing field, it is essential to obtain stable processability even when a large number of media is cut all together. The media easily cause powder fall, particularly, media such as conventional inkjet coated paper, which need to be trimmed (cut twice), cannot be used, because they may cause trouble in quality in the commercial printing field in which cut and fold are essential in processing media. Particularly, the media which need additional processing steps in contrast to the conventional printing paper, and cautious handling, cannot be used in the commercial printing field in which priority is given to processing cost, in terms of productivity.
As to these porous media, oil absorption of pigment in the coat layer has been tried to be optimized many times in order to prevent powder fall and achieve suitable gloss and image quality.
For example, Patent Literature 4 discloses cast coated paper for inkjet recording having an ink receiving layer which contains a pigment, binder and cation resin, wherein the pigment has an oil absorption of 210 ml/100 g to 330 ml/100 g, and as the binder 30 mass % to 70 mass % of polyvinyl alcohol is used relative to the pigment. Patent Literature 4 also discloses that when the oil absorption is less than 210 ml/100 g, the ink absorption is decreased, and on the other hand, when the oil absorption is more than 330 ml/100 g, the pigment absorbs the binder and the strength of the coated film is decreased. This medium has a gloss layer formed on the ink receiving layer and is treated at high cost using a cast drum so as to exhibit white paper gloss. It is extremely difficult to take a balance among ink absorption, white paper gloss and strength of the coat layer. Particularly, an inkjet media taking the balance thereof at low cost has not been achieved.
Meanwhile, when coated paper for commercial printing is subjected to typical offset-printing, matte-type paper as well as gloss-type paper can obtain relatively high image gloss. On the other hand, an inkjet medium using dye ink generally obtains gloss in an image part which is equal to that of a background part. Recently-commercialized aqueous pigment inks tend to provide lower gloss in the image part than in the background part for inkjet media. This difference in the level of gloss between the image part and background part makes the appearance of the print look strange unlike familiar commercial prints, which is a reason that the inkjet system is not used frequently for commercial printing. Additionally, in the commercial printing field, prints are sometimes get wet or used in a bare condition, and demanded to have image reliability which is much higher than that of applications for photography in which prints are stored in albums or in picture frames. In terms of this point, the aqueous dye ink is not easily used, and the aqueous pigment ink is normally preferred. In order to improve the aqueous pigment ink and image gloss, various techniques of media and recording methods for photography have been developed. However, there has not been a dominant technique for improving image gloss, which can provide high productivity (high-speed printability) and can be used in commercial printing application.
There are a great number of types of common coated paper for commercial printing having different texture, particularly the paper having different background gloss, and users can use any paper depending on desired purposes. This is because the types of paper have been increased in the whole paper industry to meet the complicated and severe requirements in texture of paper, particularly, the requirements for the background gloss and image gloss of paper by designers of publishers, in the case of commercial printing. When the inkjet system is tried to apply in the commercial printing field, a greater number of types of media must be prepared than those in photographic printing.
Moreover, the coated paper for commercial printing have different formulations of the coat layers depending on background gloss. Recently, paper making machines become larger and the amount of paper produced at once also become significantly larger. This influences production cost of paper. That is, improvement of the productivity of device as much as possible is necessary to decrease cost. In the case of common commercial printing paper, because the amount to be consumed by users are sufficiently large, it is costly advantageous that the productivity of the device is improved even by changing formulations according to each type of paper sheets. Paper sheets are easily produced into the matte and gloss type by changing the proportion of the pigment, and its production stability is high. As a result, a method of changing formulations according to qualities is preferably used.
On the other hand, as to current status of inkjet recording media, although the inkjet recording system has been popular, its market size is very smaller than that of commercial printing paper. Practically, inkjet paper is produced at a relatively low coating-speed using an off-machine coater as a coater specialized for inkjet paper in which production cost tends to be high. Generally, the first condition for producing inexpensive media is to produce the media by a large scale paper machine at high coating speed. However, in light of production of paper using these production equipment at low cost, because the amount of media demanded by user is much smaller than the amount of paper produced at a time, the amount of production is likely to be inappropriate to the productivity of the production device, or huge amount of media which does not correspond to demanded amount is produced at a time. Therefore, the cost of raw materials of the inkjet paper tends to be higher than that of coated paper for commercial printing, and in view of the current situation of the inkjet paper, it is difficult to change formulations according to types of media so as to provide various levels of background gloss. If the formulations of the inkjet paper are changed, the production efficiency is largely decreased and the production cost becomes still higher. Thus, it becomes more difficult to supply inexpensive media.
In the case of the coated paper for commercial printing, as a method in which background gloss is easily changed without changing the formulation of coat layer, the gloss is controlled in a calendering step after coating the coat layer. It may be considered that the method is also applied to the inkjet media. However, generally, inkjet gloss paper is designed to have background gloss without performing the calendaring step, while inkjet matte paper does not exhibit gloss even by calendaring and additionally does not exhibit image gloss similar to that of commercial printing.
There is another problem that when commercial porous inkjet media of either the gloss type or matte type are subjected to calender treatment, pores for absorbing ink, which have been formed by ingeniously adjusting the formulation of an ink absorption layer, become smaller in proportion to the strength of the calender treatment, and the ink absorption ability of the ink absorption layer is significantly decreased, and compatibility is changed from it before treatment. Therefore, when printing is performed under an optimized printing condition on media which have not been subjected to calender treatment, generally, image quality is also significantly decreased. Even if it is printable, different image treatment (printing condition setting) is needed. In the case of the inkjet recording, generally the image treatment is changed according to the ink absorption ability of media. Thus the number of print modes in a printer becomes vast, and it is not practical at all.
Many conventionally known techniques disclose that the oil absorption of the pigment is defined in conventional inkjet media. These objects are different from an object of the present invention, and points of views and defined ranges are different from those of the present invention. Typical examples are shown below.
Patent Literature 5 discloses an inkjet recording medium having two or more coat layers formed on a support, wherein a first coat layer, which is close to the support, contains a pigment A having a BET specific surface area of less than 100 m2/g and a pigment B having a BET specific surface area of 100 m2/g or more, the ratio of the amount of the pigment A to that of the pigment B is 9/1 to 1/9, and a second coat layer, which is far from the support, contains superfine particles selected from silica having an average particle diameter of 0.7 μm or less, alumina and alumina hydrate, and water soluble resin. This inkjet recording medium is to attain a photographic medium or medium for printing proof, by cast method, and has the outermost layer for receiving ink which needs to contain superfine particles having a particle diameter of 0.7 μm or less, thereby being very expensive medium. Additionally, Patent Literature 5 seeks for function in the undercoat layer, and obviously, background gloss and image gloss cannot be obtained as the outermost layer.
Patent Literature 6 discloses an inkjet recording paper having an ink receiving layer formed to be the coated amount of 2 g/m2 to 10 g/m2, wherein the ink receiving layer contains a pigment which is synthetic amorphous silica having an oil absorption of 150 ml/100 g to 250 ml/100 g. This paper is aimed at a non-aqueous pigment ink. An aqueous ink is not suitable for this inkjet recording paper, thereby obtaining poor image quality.
Patent Literature 7 discloses an inkjet recording medium containing at least one ink receiving layer formed on a support, wherein the ink receiving layer mainly contains a pigment and hydrophilic binder, and the pigment has an oil absorption of 100 ml/100 g to 350 ml/100 g, and further contains 20 parts by mass or less of a calcium phosphate compound per 100 parts by mass of the pigment, and the calcium phosphate compound has a solubility in water at 30° C. of 3 g/100 g or less. This inkjet recording medium is excellent in ink absorption and color developing property of image using any of a dye ink and pigment ink. However, this system is related to an inkjet matte paper, thus background gloss and image gloss cannot be obtained.
Patent Literature 8 discloses an inkjet recording paper having an ink receiving cover layer formed on a surface of paper which mainly contains cellulose pulp, wherein the ink receiving cover layer mainly contains 50 wt % to 90 wt % of a pigment having an oil absorption (JIS K 5101) of 50 ml/100 g or more and 10 wt % to 50 wt % of a water soluble adhesive containing 3 wt % to 30 wt % of a modified starch in the solid content of total water soluble adhesive, and the water soluble adhesive contains a water-soluble polymer which is at least one selected from polyvinyl alcohol, modified polyvinyl alcohol and casein, and modified starch. According to Patent Literature 8, the uniformity of coating and uniformity of ink absorption can be significantly improved, but the water soluble adhesive must be used, and the proportion of the total amount of pigment cannot be 90 wt % or more due to powder fall. Thus, the coating solution has high viscosity, and it is disadvantageous to coating at high speed.
In addition to the above description, in the media that is made inkjet suitable, a cationic additive or a sizing agent is used to improve the fixing ability of the colorants and dyes and the pH of the paper surface is brought close to acidic one. Thus, an additive as described in Patent Literature 9 is used, cationic fine particles as described in Patent Literature 10 are used, and the filler surface is treated with a cationic resin, as described in Patent Literature 11. In most cases, an acidic dye demonstrating anionic properties is used for dye inkjet inks. Because the fixing ability of a dye can be increased by bonding a sulfonic group or the like of an acidic dye with a cationic substance contained in the ink, such technology is presently widely used. Coloring pigments of pigment inkjet inks are also often anionic, and the fixing ability can be improved by the same mechanism. For these reasons, in the case of inkjet paper, a treatment that decreases the pH of paper surface (shifts it to acidic side) to ensure excellent preservation of inkjet image is actively conducted, and practically all media designed for inkjet applications has a paper surface pH of 7 or less.
However, inks for commercial printing such as offset printing often employ an oil with a very high boiling point as a solvent and, by contrast with inkjet inks, the fixation of image after printing is realized by oxidation and polymerization of the solvent. In offset printing, fast fixing by drying (oxidation polymerization reaction) is also desired from the standpoint of operability. For this reason, a drying (polymerization) enhancer called “drier” is often added to the ink. Because the drier forms a precipitate of metal ion components under an acidic atmosphere and becomes ineffective, it is known that drying property is decreased significantly in the case of paper having a low pH of paper surface such as inkjet media. In addition, it is well known that dampening water (H liquid) that is used in offset printing sometimes react with cationic substances of inkjet media, thereby destroying the hydrophilic-hydrophobic balance of the printing plate and easily causing printing defects such as image smear. It is also for such technological reasons, rather than only because of cost factors, that the offset printing technology is very difficult to apply to the inkjet media. Although the media are inkjet specialized media, offset printability is an essential function when they are used for commercial printing, thus the offset printability is necessary to be improved.
For the above reasons, in order to apply the inkjet recording system to the commercial printing field, many technical improvements are necessary, such as a technique of producing media having various texture and gloss at low cost, a technique of making compatibility closer to each of various types of media, a technique of attaining image gloss in a matte texture at low cost, improvement of post-treatment properties such as prevention of powder fall, offset printability, and the like.
The inventors of the present invention have studied an inkjet recording method which enables to obtain texture close to the commercial printing at low cost, for the purpose of applying the inkjet recording method using an aqueous pigment in the commercial printing field.
In order to resolve the above-mentioned problems, Patent Literature 12 suggests a low-cost method for recording images on commercial-grade paper by combining a pigment ink with high penetration ability and a recording medium that, by contrast with the conventional media, has low ink absorption ability. With this method, only a solvent (water or an organic solvent) forming the ink is selectively caused to infiltrate into the substrate by recording using a small amount of pigment ink with ultrahigh penetration ability on a recording medium having provided thereon a coat layer for inhibiting ink absorption (penetration) so that the pigment contained in the ink does not infiltrate intensively into the medium, and the colorant (pigment) contained in the ink can be caused to remain on the medium surface with good efficiency, without using any special material such as cation fixing agent. As a result, both a sufficient density and drying ability can be realized with a small amount of ink. Moreover, because the colorant contained in the ink is caused to remain effectively on the medium surface, high transparency of the layer that was a necessary function in the conventional recording media becomes unnecessary. Therefore, the degree of freedom relating to material configuration of the coat layer can be greatly increased. By applying this method, it is possible to perform inkjet recording even on paper with low ink absorption ability such as commercial printing paper or publication printing paper. However, with this method, it has been difficult to print image quality as high as that of conventional offset printing directly on commercial printing paper at a commercially practical speed. Because these paper sheets have poor drying property, set-off occurs by stacking the paper immediately after printed at high speed and output. Therefore, there are abundantly many restrictions in use of those paper sheets when they are used for business purpose, such as printing business.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-212327    Patent Literature 2: JP-A No. 11-078225    Patent Literature 3: JP-A No. 2005-288696    Patent Literature 4: JP-A No. 2006-240017    Patent Literature 5: JP-A No. 2006-231914    Patent Literature 6: JP-A No. 2006-240270    Patent Literature 7: JP-A No. 2006-218697    Patent Literature 8: Japanese Patent (JP-B) No. 3074743    Patent Literature 9: JP-B No. 3349803    Patent Literature 10: JP-A No. 2003-080837    Patent Literature 11: JP-A No. 2006-321978    Patent Literature 12: JP-A No. 2007-144975